INTRODUCTION The flower meristem in angiosperms gives rise to floral organs in numbers and patterns characteristic of an individual species. In Arabidopsis thaliana, floral meristems initiate four concentric rings, or whorls, of organs, with 4 sepals in the first (outer) whorl, 4 petals in the second whorl, 6 stamens in the third whorl, and 2 fused carpels in the central gynoecium. The organs arise in defined positions as well, presumably due to stereotypic changes in cell division patterns at organ initiation sites that give rise to organ primordia, which then undergo further cell divisions and cell differentiation to form the mature organ. The mechanisms by which the sites of organ initiation in the floral meristem are determined is unknown. One recent approach to studying this problem has been to isolate mutations that change the number and position of organs. Mutations in the CLAVATA1 (CLV1) and CLAVATA3 (CLV3) genes increase organ number in all 4 floral whorls, especially the inner whorls, as well as increasing the total number of whorls (Leyser and Furner, 1992; Clark et al., 1993, 1995; Crone and Lord, 1993; Alvarez and Smyth, 1994). Mutations in the PERIANTHIA (PAN) gene cause an increase in sepal and petal number and a slight decrease in stamen number (Running and Meyerowitz, 1996). Mutations in the FASCIATA1 (FAS1) and FASCIATA2 (FAS2) genes show a variable organ number change in the first three whorls of the flower (Leyser and Furner, 1992). Mutations in TOUSLED (TSL; Roe et al., 1993, 1997), REVOLUTA (REV; Talbert et al., 1995), SHOOT MERISTEMLESS (STM; Barton and Poethig, 1993; Clark et al., 1996; Endrizzi et al., 1996), AINTEGUMENTA (ANT; Elliot et al., 1996; Klucher et al., 1996) and WUSCHEL (WUS; Laux et al., 1996) lead to decreased organ numbers in all 4 whorls. In clv1 and clv3 mutants, the increase in organ and whorl number is correlated with specific changes in floral meristem size at the time of organ initiation (Clark et al., 1993, 1995; Alvarez and Smyth, 1994). Defects in the apical and/or floral meristem size or structure have also been reported in fas1, fas2, stm and wus mutants (Leyser and Furner, 1992; Barton and Poethig, 1993; Clark et al., 1996; Laux et al., 1996; Endrizzi et al., 1996). In this paper we describe the effects of mutations in WIGGUM (WIG), a gene required for maintenance of apical and floral meristem structure, floral organ number, and organ patterning. wig mutant plants show an increase in organ number similar to those seen in pan mutants, with extra sepals and petals, and some effects on stamen and carpel number as well. However, detailed studies of the wig mutant phenotype, as well as analysis of double mutant combinations of wig with other mutants, suggest that WIG acts in manners distinct from those of previously described genes, and uncovers a novel 2545 Development 125, 2545-2553 (1998) Printed in Great Britain © The Company of Biologists Limited 1998 DEV0163 The study of cell division control within developing tissues is central to understanding the processes of pattern formation. The floral meristem of angiosperms gives rise to floral organs in a particular number and pattern. Despite its critical role, little is known about how cell division is controlled in the floral meristem, and few genes involved have been identified. We describe the phenotypic effects of mutations in WIGGUM, a gene required for control of cell proliferation in the floral and apical meristem of Arabidopsis thaliana. wiggum flowers contain more organs, especially sepals and petals, than found in wild-type flowers. This organ number phenotype correlates with specific size changes in the early floral meristem, preceding organ initiation. Genetic studies suggest that WIGGUM acts on a similar process but in a separate pathway than the CLAVATA1 and CLAVATA3 genes in meristem size regulation, and reveal interactions with other genes affecting meristem structure and identity. Analysis of double mutant phenotypes also reveals a role for WIGGUM in apical meristem function. We propose that WIGGUM plays a role in restricting cell division relative to cellular differentiation in specific regions of the apical and floral meristems. Key words: Arabidopsis, WIGGUM (WIG), Floral meristem, Flower development, Pattern formation SUMMARY The WIGGUM gene is required for proper regulation of floral meristem size in Arabidopsis Mark P. Running*, Jennifer C. Fletcher and Elliot M. Meyerowitz † California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA *Present address: University of California Berkeley, USDA-ARS Plant Gene Expression Center, 800 Buchanon St, Albany, CA 94710, USA † Author for correspondence (e-mail: meyerow@cco.caltech.edu) Accepted 8 May; published on WWW 23 June 1998